The direct reduction of iron oxide, such as pellets or lump ore, to metallic iron in the solid state has in recent years become a commercial reality in many direct reduction plants throughout the world. The combined annual capacity of these plants currently in operation or under construction is in excess of 15 million metric tons of direct reduced iron product, which is used primarily for feedstock in electric arc steelmaking furnaces. The world demand for additional direct reduced iron is projected to increase at a substantial rate for many years to satisfy a growing world need for such feedstock, as additional electric arc furnace steelmaking plants are constructed.
The majority of the plants producing direct reduced iron utilize natural gas as the source of reductant. The natural gas is reformed to produce the reductants CO and H.sub.2. A few plants utilize coal as the source of reductant in rotary kiln processes, such as the SL/RN process, which directly react coal in-situ in the kiln without separately gasifying the coal to CO and H.sub.2. The rotary kiln processes have an inherent coal utilization inefficiency in that approximately two-thirds of the coal is burned in the kiln to supply heat and only one-third is used to supply the reducing gas for direct reduction. This inefficiency results in a coal requirement of 5.0 to 6.0 Gcals (Gigacalories) per metric ton of direct reduced iron produced. This is in contrast to 3.0 to 3.5 Gcals of natural gas required per metric ton of direct reduced iron produced in the more efficient natural gas processes such as the Midrex, Purofer or Armco processes.
There are many processes such as set forth in U.S. Pat. No. 3,853,538, not yet commercialized, which gasify coal by partial oxidation with oxygen and steam to produce a gas which is then utilized in different manners in the direct reduction of iron. The principal reason none of these processes has been commercialized is either the process is too complex or impractical for commercialization, or the coal requirements are too high. The basic problem which leads to an impractical process or to a high total coal requirement is that the hot gas from the coal gasifier is too low in reductants (CO plus H.sub.2) relative to oxidants (CO.sub.2 plus H.sub.2 O vapor) to be directly utilized efficiently in the direct reduction of iron.
In the present invention, the hot gas from the coal gasifier is tempered with upgraded spent reducing gas from the reduction furnace and desulfurized by reaction with a sulfur acceptor, preferably lime, to produce a hot desulfurized gas. A second stream of upgraded spent reducing gas is heated by the burning of spent reducing gas and is mixed with the hot desulfurized gas to produce hot reducing gas for the reduction furnace. The invented process achieves high thermal efficiency and makes practical the commercial realization of direct reduction of iron using coal gasification as the source of reductant with the improved coal gasification technology now under development. The present invention requires approximately 3.1 Gcals of coal to be gasified and about 0.4 Gcals of coal for generating electricity at 30% conversion efficiency to produce oxygen for the gasification, for a total coal requirement of approximately 3.5 Gcals per metric ton of direct reduced iron.